The present invention relates to an expiration valve of the type intended to be connected to the expiratory outlet of apparatus for artificial positive pressure ventilation of a patient's lungs, in order to create a positive pressure which is retained in the patient's lungs when expiration, or exhalation, has terminated.
This form of artificial ventilation of a patient's lungs, where the patient exhales to a controlled positive pressure level and not to the atmospheric pressure, is known as "PEEP" ventilation, PEEP being an acronym for Positive End-Expiratory Pressure ventilation.
Various kinds of PEEP valves that permit such ventilation to be carried out are known in the art.
The simplest kind of PEEP valve consists of a flexible tube one end of which is attached to the expiratory outlet of the apparatus and the other end of which is immersed in water or other fluid, so that air expired by a patient has to displace the water column present in the tube in order to escape and rise to the surface of the water in the form of bubbles. The end-expiratory pressure thus becomes equal to the depth to which the tube is immersed in the water. Such an arrangement is, however, only suitable for use in conditions in which a receptacle for the fluid can be firmly supported and kept steady.
Other arrangements are known in which the expiratory outlet of ventilating apparatus is connected to a valve opening leading to a valve seat covered by a diaphragm, the downstream side of which is loaded by an adjustable pneumatic pressure, so that expiration via the valve can only occur as long as the expiratory pressure is capable of lifting the diaphragm free of the valve seat against the downstream pneumatic pressure.
Such valve arrangements can, however, only be used in circumstances in which an accurately controllable and adjustable course of pneumatic pressure is available for loading the diaphragm, e.g. from the control system of a respirator.
Furthermore, spring-loaded PEEP valves are known, such valves being composed of an inlet adapted for connection to the expiratory outlet of ventilation apparatus and leading to a valve seat which is covered on its downstream side by a spring-loaded valve disc, so that expiration only takes place via such a valve if the expiratory pressure is capable of opening the valve against the spring force acting on the valve disc.
Spring-loaded PEEP valves have the advantage of being compact and usable without requiring pneumatic pressure or a fluid receptacle. However, known designs have had the disadvantage that they tend to vibrate during expiration, reducing the accuracy of the valve's setting capability and producing an unacceptably high noise level.